Passive Q-switching allows narrowing of the linewidth of a laser oscillator and can in principle provide the generation of single-frequency pulses. Typically, single-frequency laser pulses are generated using a passively Q-switched standing-wave cavity having a relatively short length. For example, solid state Nd-lasers passively Q-switched using saturable semiconductor structures can generate single-frequency laser pulses as short as ˜40 ps. Also Nd-lasers can be passively Q-switched by Cr4+-doped materials (such as Cr4+:YAG) or color center materials (such as LiF:F2− crystals); however, the laser pulsewidth in this case can be much longer-ranging from ˜1 s to ˜10 s of ns. For generation of single-frequency pulses, it is advantageous to have a short laser cavity so that the longitudinal mode spacing is maximized and the number of longitudinal modes matching the spectral gain curve of the laser active medium is minimized. Short length of the laser cavity facilitates the generation of short laser pulses. However, the time-bandwidth relationship dictates that the laser bandwidth increases as the laser pulses are shortened. This makes short, single-frequency laser pulses undesirable for various applications where the signal of interest has a small frequency shift, e.g., coherent LIDAR applications, or the laser source is used to excite various subjects with extremely narrow spectral lines, e.g., CO2 detection. The linewidth requirements can be satisfied for a pulsed laser by increasing the laser pulsewidth to hundreds of ns (while maintaining the single longitudinal mode operation), which decreases its transform-limited spectral linewidth.
FIG. 1A shows a prior art Q-switched oscillator using a standing-wave resonator. A pump source 90 provides pump light 100, which enters the resonator through a dichroic mirror 101, which is highly transmissive at the pump wavelength and highly reflective at the laser wavelength. The pump laser is incident on the gain medium/laser element 103 and is fully or partially absorbed in the laser element. The laser emission 105 escapes through the output coupler 102, which is a partial reflector at the laser wavelength. To achieve a pulsed regime of operation a Q-switch element 104 can be inserted in the cavity. Such Q-switching elements can be passive (e.g., a saturable absorber) or active (e.g., an electro-optic or acousto-optic Q-switch). The output coupler 102 can be a mirror, a surface grating, or a thick holographic (volume) grating.
Standing wave resonators typically produce short laser pulses (40 ps to ˜80 ns). An inherent disadvantage of a standing wave resonator is spatial hole-burning, which can lead to generation of other longitudinal modes. To overcome the limitations of the standing-wave oscillator, one can consider a ring resonator design that can eliminate spatial hole-burning and increase the laser pulsewidth due to the fact that the laser beam passes only one time through the gain medium per roundtrip.
FIG. 1B shows a prior art ring resonator. Typically, ring resonators are based on a 4-mirror design with two dichroic concave mirrors 101 and two flat mirrors 102, 106. One of two flat mirrors serves as an output coupler 102. An “optical diode” provides unidirectional operation. An optical diode can include a non-reciprocal Faraday rotator and a reciprocal rotator (such as half-wave plate).
To achieve laser operation at a specific wavelength and decrease the laser linewidth, a narrow-bandwidth element can be incorporated into the cavity. For example, a surface grating or an intracavity etalon (or a pair of etalons) can be used. However, intracavity etalons are extremely temperature and angle-alignment sensitive elements and make the design of wavelength-stable oscillators difficult. Surface gratings have relatively low laser damage threshold and require intra-cavity beam expansion to increase spectral resolution.
It, therefore, would be advantageous to develop a ring resonator that can utilize a thick Bragg grating (TBG) as a narrow-bandwidth reflector.